In conventional stamping processes, it is often desirable to hold a blank sheet of metal about its peripheral margin during forming of a stamped component. In such processes, it is known to provide a female form die mounted to a movable upper platen of a press, a male form punch fixedly mounted to a fixed lower platen of the press, and a binder ring displaceably mounted to the press. The binder ring encircles the form punch and is positioned vertically flush with respect thereto, such that the blank sheet of metal can be laid flat across the binder ring and the form punch. In operation, the upper platen strokes downwardly, wherein the form die pinches the peripheral margin of the blank sheet of metal against the binder ring and displaces the binder ring downwardly so as to stretch the blank sheet of metal over the form punch to produce the stamped component. The binder ring and its displaceable mounting structure are often known as a die cushion.
Die cushion apparatuses are widely used in conventional stamping processes for manufacturing automotive body panels. Die cushion apparatuses are often integrated into a press machine and may be used in stripping finished parts from a punch or a die, and in actuating ejector pins that push a finished part from a die cavity or from a die punch. Also, in double-action presses, die cushion apparatuses are often used to keep a sheet metal blank flat, to hold the blank to shape, or to prevent the blank from slipping and distorting during drawing. Moreover, in single-action presses, die cushion apparatuses enable relatively uniform blankholding force. In a single-action press using a die cushion apparatus, a die punch is mounted on a lower plate of the die cushion apparatus and a form die is mounted above an upper plate of the die cushion apparatus. Cushion cylinders are mounted between the upper and lower plates of the die cushion apparatus for applying an upward bias force against the upper plate to hold an outer portion of a sheet metal blank against the form die during a downward stroke of the form die.
Die cushion apparatuses can also be mounted to a press machine as a self-contained assembly that is adapted for use in hot stretch-forming processes for manufacturing automotive body panels. Hot stretch-forming processes may include super-plastic-forming and quick-plastic-forming processes in which a sheet metal blank is pinched at its periphery between complementary surfaces of opposed die members of a press machine. For example, a lower platen of the press machine may carry a die cushion assembly having upper and lower plates connected by guidance cylinders, which are fixed to the upper and lower plates at the corners thereof. In any case, a male punch is typically mounted to the lower plate of the die cushion assembly and a binder ring is typically carried by the upper plate of the die cushion assembly. An upper platen of the press may carry a female preform tool, wherein the sheet metal blank becomes pinched between the preform tool and the binder ring to form a pressure tight seal between the sheet metal blank and the preform tool. Accordingly, the preform tool also serves as a pressure flask or vessel. The preform tool design typically includes a cavity for clearance fit operation with the male punch and the design minimizes the volume of the cavity between the preform tool and male punch. This reduces the required super-plastic-forming pressure to about 400 psi and improves radiant heating of the sheet metal blank by minimizing the distance therefrom to the preform tool.
To provide the heat necessary for stretch-forming, electrical heating elements are typically located in the female preform tool and binder ring as well as in the male punch. The electrical heating elements primarily heat the male and female tooling, but also tend to incidentally heat the upper and lower plates of the die cushion assembly. Unfortunately, however, the temperatures of the upper plate and the lower plate of the die cushion tend to be significantly different due to differences in the heat generated by the electrical resistance heating elements or by differences in residual heat generated during the forming process. Such differences in temperature between the upper and lower plates tend to yield different thermal expansion characteristics thereof. For example, forming process temperatures tend to be greatest near the upper plate, thereby leading to greater displacement thereof due to relatively greater thermal expansion of the upper plate compared to the lower plate. In other words, the upper plate tends to expand outwardly to a greater degree than the lower plate, and the upper plate thereby urges the upper end of the guidance cylinders in a direction laterally away from the lower end of the guidance cylinders, thereby leading to binding of the guidance cylinders and inoperability of the die cushion assembly.
Thus, there is a need to eliminate binding of guidance cylinders between upper and lower plates of a die cushion assembly.